1. Field of the Invention
The present invention relates to a photocurable resin composition excellent in shaping and form retention, which is particularly suitable for formation of a finely embossed surface structure on an optical article, a method of forming a finely embossed pattern by using the composition, a finely embossed pattern receiptor (for example, a finely embossed pattern-forming sheet or a finely embossed pattern transfer sheet), an optical article and a stamper.
2. Description of the Related Art
Conventionally, a photocurable resin composition (coating) is applied onto a support such as polyester film to form a photocurable resin layer thereon, and then the surface of this photocurable resin layer is provided with various finely embossed patterns capable of exhibiting optical functions and exposed to active energy rays such as UV rays or electron rays to cure the resin layer, followed by laminating a metal-deposited layer or a layer having different refractive index on the surface of the formed embossed pattern, to form an optical article such as a diffraction grating or a relief hologram.
The optical article having a finely embossed pattern thereon is required of general properties such as embossed pattern accuracy, transparency, strength, resistance to wearing, heat resistance, water resistance, chemical resistance, adhesion to a substrate, flexibility or the like depending on its purpose. Accordingly, a photocurable resin composition forming the optical article should, first of all, satisfy these performance requirements.
The conventionally known method of forming a fine relief surface structure of an optical article includes the so-called 2P method (photo polymer method) for formation of e.g. a relief hologram wherein a press stamper (referred to hereinafter as “stamper”) prepared from a master hologram having a desired finely embossed pattern formed thereon is pressure-welded with a liquid photocurable resin layer formed by applying a liquid photocurable resin composition onto a transparent support such as polyester film, and simultaneously irradiated at the side of the support with light to cure the resin layer, followed by removing the stamper therefrom.
For example, Japanese examined Patent Publication No. 4-5681 describes a photocurable resin composition usable in the 2P method, which comprises an oligomer having a hard segment (H), a soft segment (S) and a crosslinkable-terminal segment (E). This oligomer composition has the following advantages: an embossed pattern from a mold can be accurately reproduced; the composition is suitable for mass production because embossing can be successively carried out using one press stamper to produce a large number of articles; it is also excellent in releasability after curing by exposure to light and thus suitable for mass production in this respect too; optical articles excellent in strength, flexibility, optical transparency or uniformity, and resistance to common solvent can be obtained.
In the 2P method, however, there are the following problems: because the backside of the support is irradiated with light, curing is insufficient when the support is light-absorptive; the process is time-consuming because the stamper cannot be removed until curing is completed; because the photocurable resin layer is liquid, bubbles are easily introduced between the support and the stamper, to cause defects; and when the stamper is removed from the cured resin layer, the surface of the resin layer can string thus causing surface roughness.
As another method, there is a method wherein a photocurable resin composition which is highly viscous or solid at room temperature is applied onto a support to form a photocurable resin layer, then a stamper is pressure-welded therewith and then removed, and thereafter the photocurable resin layer is cured by irradiation with light (Japanese examined Patent Publication No. 5-46063, Japanese examined Patent Publication No. 6-85103).
In this method, there are the following advantages: the photocurable resin layer is irradiated directly with light, so the resin can be sufficiently cured; since the photocurable resin composition is highly viscous or solid, the support after coating can be rolled and stored or conveyed, that is, coating and reproduction can be carried out in separate processes; after the stamper is removed, the resin can be irradiated with light, so pressing with the stamper and curing can be carried out in separate processes; because bubbles are hardly introduced between the photocurable resin layer and the stamper, accurate patterning can be carried out; and when the stamper is removed from the photocurable resin layer, the surface of the resin layer hardly strings.
In the method of using the photocurable resin composition which is highly viscous or solid at room temperature, however, there are the following problems: because the stamper is removed from the resin layer in a un-cured state, apart of the photocurable resin composition easily adheres to and remains on the cavity of the stamper (phenomenon of partial remaining on the plate); accurate shaping cannot be achieved because as an embossed pattern to be reproduced is finer, the photocurable resin composition is more hardly filled in the cavity of the stamper; and after the stamper is removed, the resin layer in a un-cured state is irradiated with light, so the pattern can be easily deformed until curing is finished.
Japanese Patent Application Laid-open No. 2000-63459 describes a photocurable resin composition comprising a bulky group-containing urethane-modified acrylic resin and a release agent as essential ingredients. This photocurable resin composition comprises an urethane-modified acrylic polymer having crosslinkable groups, and has advantages such as accurate reproduction of an embossed pattern with a mold, suitability for mass production by virtue of excellent releasability from the mold even before curing, and other excellent general properties such as transparency, strength, resistance to wearing, heat resistance, water resistance, chemical resistance, adhesion to a base meterial, flexibility etc. Further, there is another advantage of excellent fitting after curing to the flex, expansion and contraction of the substrate.
According to the invention in Japanese Patent Application Laid-open No. 2000-63459 mentioned above, the photocurable resin composition comprising urethane-modified acrylic resin is applied onto a substrate and dried if necessary to form a photocurable resin layer, and a press stamper is pressed thereon (embossing), to form the embossed pattern of the press stamper (correctly a pattern complementary to the embossed pattern of the press stamper) on the resin layer, and thereafter the press stamper is removed from the resin layer, and then the resin is cured by exposure to light to fix the embossed pattern. Accordingly, embossing can be successively carried out without using the press stamper in the light exposure process, while light exposure is not hindered by the press stamper, and thus uniform and sufficient light exposure can be carried out in a short time.
When the stamper is removed from the photocurable resin layer in a un-cured state, a part of the photocurable resin adheres to and remains on the cavity of the stamper (phenomenon of partial remaining on the plate), thus causing a problem that successive and accurate embossing is hardly carried out using the same stamper. Against this problem, the photocurable resin composition described in the above Japanese Patent Application Laid-open No. 2000-63459 has the advantage that the photocurable resin composition is excellent in releasability even in a un-cured state and hardly adheres to the cavity of the stamper.
However, if as shown in the invention in Japanese Patent Application Laid-open No. 2000-63459 supra, the photocurable resin composition is applied onto a substrate and dried if necessary to form a layer on which a stamper is then pressed, the flexibility and fluidity of the photocurable resin composition can be insufficient so that the resin may not sufficiently spread throughout the cavity space, thus failing to accurately form the desired finely embossed pattern.
To improve the ability of a photocurable resin composition to be shaped (easiness of shaping and accuracy of shaping), there is a method of pressing a stamper with a layer comprising a photocurable resin composition formed on a support (that is, in a stamping or embossing process) wherein pressing is carried out under heating at a relatively high temperature and/or the speed of feeding the support is decreased to prolong the pressing time. However, there are problems such as damage to the stamper and deterioration in the support by increasing the pressing temperature and a drop in production efficiency by decreasing the speed of endowing a finely embossed pattern.
In an alternative method, a photocurable resin composition easily softened at room temperature can be used to improve easiness of shaping of a layer comprising the photocurable resin composition. However, if the photocurable resin composition easily softened at room temperature is used, a roll produced by rolling its photocurable resin layer formed on a support is sticky and liable to blocking, so there is inconvenience that it cannot be stored.
Further, even if the finely embossed pattern on the stamper can be accurately formed on the photocurable resin layer, the finely embossed pattern formed on the photocurable resin layer, when removed in a un-cured state from the stamper and exposed to light, is rounded and deformed to collapse its shape (called “sagging”) due to the elasticity of the resin itself during the process of forming the finely embossed pattern. Such collapse of the shape is attributable mainly to e.g. heat transferred from the stamper to the un-cured photocurable resin layer upon heat embossing, heat upon UV irradiation of the photocurable resin layer having the finely embossed pattern formed thereon, etc.
For accurate and efficient reproduction of a finely embossed pattern having optical functions by using the photocurable resin composition, it is therefore necessary that the photocurable resin composition in the form of a layer containing less or no solvent has the ability to form a film sufficiently, to be shaped by fitting completely to the cavity of the stamper and to retain the sharp form formed by the press stamper even in the process after removal of the stamper from the photocurable resin.
In recent years, optical finely embossed patterns are becoming very complicate as precision processing technology is improved. These complicate optical finely embossed patterns can be used to form holograms of complicate design, information-recording elements, or optical elements highly regulating reflection, transmission, scattering, polarization, convergence or interference of whole rays and/or a light at a specific wavelength. However, such complicate and finely embossed patterns have narrower embossed pitches and/or deeper embossed portions than those of relatively simple holograms in the prior-art, thus making accurate reproduction more difficult, and satisfactory functions cannot be exhibited without reproduction of higher accuracy than in reproduction of relatively simple holograms in the prior-art.
Such complicate optical finely embossed patterns, even using the resin composition described in Japanese Patent Application Laid-open No. 2000-63459 supra, can not avoid the problems of the partial remaining of resin, the shaping ability of resin and the collapse of the pattern, and thus accurate reproduction is very difficult.
On one hand, Japanese Patent Application Laid-open No. 7-502128 describes an optical element having a surface structure obtained by embossing a transparent composite material having a 3-dimensional skeleton of inorganic or organic modified inorganic components in the form of nano-scale particles in a polymer matrix. According to this prior-art invention, shrink in the procedures of embossing and curing can be minimized. In the present invention, however, a photo- or thermosetting resin composition containing inorganic or organic modified inorganic components in the form of nano-scale particles is applied onto a substrate to form a photo- or thermosetting resin layer thereon, then cured by light exposure or heating while an embossing device is pressed on the resin layer, and subjected if necessary to pre-curing for increasing viscosity before attachment of the embossing device and/or post-curing for completion of copolymerization after removal of the embossing device. In this prior-art invention, basically the photo- or thermosetting resin layer is cured while the embossing device is pressed thereon, and it is not described therein that when the resin layer being still in the un-cured state is removed from the embossing device, collapse of the shape occurs due to the elasticity of the resin itself.